1. Field of the Invention
This invention relates to molded case circuit breakers and more particularly to an interlock which prevents the circuit breaker operating handle from being placed in an off position during a condition when the main contacts become welded together.
2. Description of the Prior Art
Molded case circuit breakers are generally old and well known in the art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 4,489,295; 4,638,277; 4,656,444 and 4,679,018. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and relatively high level short circuit condition. An overload condition is normally about 200-300 percent of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000 percent or more of the nominal current rating of the circuit breaker.
Molded case circuit breakers include at least one pair of separable main contacts which may be operated either manually by way of an operating handle disposed on the outside of the case or automatically in response to an overcurrent condition. In the automatic mode of operation, the contacts may be opened by an operating mechanism, controlled by a trip unit, or by magnetic repulsion forces generated between the stationary and movable contacts during relatively high levels of overcurrent.
In one automatic mode of operation, the contact assemblies for all poles are tripped together by the trip unit which, in turn, actuates the mechanical operating mechanism causing the circuit breaker to trip. In the other automatic mode of operation, the contact arm assemblies are disengaged from the mechanical operating mechanism and are blown open by magnetic repulsion forces.
Under certain abnormal operating conditions, the separable main contacts may become fused or welded together due to the magnitude of the overcurrent and the time period that such an overcurrent flows through the main contacts. One such abnormal operating condition can occur when there is a failure or a partial failure (e.g., binding) of any of the components within the circuit breaker that relate to the tripping operation. More specifically, a failure of either the trip unit or the operating mechanism can result in either a failure or a delay of the main contacts in being separated during an overcurrent condition. This can result in heating of the main contacts to the point where they become at least partially molten and fuse or weld together.
After a predetermined time period, an upstream radial feed breaker will generally clear the overcurrent condition to allow the damaged circuit breaker to be removed to allow the main contacts to be replaced. However, maintenance personnel will be initially unaware that a problem with the circuit breaker exists. Accordingly, maintenance personnel will manually trip the circuit breaker (e.g., place the operating handle in the off position). This results in the operating handle indicating an incorrect status of the main contacts. More specifically, circuit breakers normally have three positions; an on position, an off position and a trip position. The on position normally indicates that the main contacts are closed. The off position normally indicates that the main contacts have been opened by the circuit breaker operating handle. The trip condition normally indicates the main contacts have been opened automatically by the trip unit.
The problem is even further complicated when the circuit breaker is padlocked in the off position when the main contacts are welded together. Accordingly, it is desirable to prevent the operator from placing and paclocking the operating handle in an off position when the main contacts are welded together.